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Electronic Amplifiers Amplifier: activ three-port network that delivers to the output a signal xo(t) (voltage or current) with the same shape as the input signal xi(t) and can provide greater power on an adequate load. Linear circuit: x0 proportional with xi A – amplification, gain xo (t ) Axi (t ) A<0 inverting A>0 non-inverting 1/13 The Amplifier Supply dc voltage and/or current sources more frequently – dc voltage sources Single source supply Two source supply (symmetric differential) 2/13 Power transfer and power balance the average power of the output signal Pout is greater than the average power of the input signal Pin. the excess of the output power is taken from the supply sources Psupply+Pin=Pout+Pdissip Psupply Pout+Pdissip efficiency η =Pout/Psupply a step up transformer is not an amplifier 3/13 Amplifier types 4/13 VTC for a voltage-to-voltage noninverting amplifier, symmetric differential supply amplification (active) region: VOL VOH ; vI ; Av Av vO VOL ;VOH ideal amplifier: VOL=-VPS VOH=+VPS general-purpose OA vO (VPS 1V...2V; VPS 1V...2V) rail-to-rail OA: vO VPS ;VPS 5/13 Signal transfer Remark: input signal - low enough for the amplifier to work in a linear region around the OP: small signal approximation 6/13 Amplifier models two-port network: it consider explicitly only the behavior to the input and output ports and input-output transfer for signal the signal valid irrespective of the internal complexity of the amplifiers valid in the bandpass frequency domain Linear controlled sources active two-port network – only one finite, non-zero parameter : dforward transfer parameter (gain) the output signal is controlled by the input signal pseudo-sources Example: VCVS vO = avvi 7/13 Modeling the voltage amplifier vo av vi Ri – draws current from the input signal source Ro – deteriorates the output voltage in the presence of load (voltage divider) vo Av vs Ri RL Av av Rs Ri RL Ro Ideal amplifier ? 8/13 Av is closer to the open circuit gain av, when one can reduce the voltage losses to the input (across Rs) and to the output (across Ro) Ri>>Rs – the source voltage appears to the input of the amplifier vi vs Ro<<RL - the voltage of the controlled voltage appear to the output vo avvi ideal voltage amplifier Ri = ∞; Ro = 0 9/13 Determining the amplifier performances gain (forward transfer factor) input resistance output resistance Gain • analysis the circuit using theorems and electrical circuit relations (Kirchhoff, Ohm, etc.) and equations describing the operation of the active devices express the output signal as a function of the input signal and compute the gain Input resistance vi Ri ii 10/13 Output resistance Set the input signal source to zero 1. Connect to the output a test source vtest Ro itest 2. open short-circuit Ro vo ,open io ,sc 11/13 Frequency response analyze the equivalent model of the amplifier including capacitive components too, by their complex impedances 1/jωC Complex transfer function for the gain: vo ( j ) A( j ) vi ( j ) input and output impedances are replaced by their complex counterparts Z i ( j ) Zo j Band-pass amplifier 12/13 the decrease of the gain to high frequency - internal capacitances (parasitic) of the active devices the decrease of the gain to low frequency – coupling / decoupling capacitors (usual fractions – tens of F) dc coupling (direct) LPF capacitive coupling at input capacitive coupling of two amplifier stages 13/13